correct gpio.txt typos

Corrected sysfs gpio chip node name and fixed punctuation.

Signed-off-by: Dominik D. Geyer <dominik.geyer@gmx.de>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>